Magnesium batteries have received attention as a type of post lithium-ion battery because of their potential advantages in cost and capacity. Among the host candidates for magnesium batteries, orthorhombic α­-V₂O₅ is one of the most studied materials. The enhanced electrochemical activity of Mg intercalation into α-V₂O₅ in a wet organic electrolyte has been an interesting yet puzzling phenomenon for the last two decades because it has been unclear which species were really inserted into the host structure during the electrochemical reduction (magnesium-insertion) reaction. Studies by several groups during the last two decades have demonstrated that water plays some important roles in getting higher capacity.^1-4 Very recently, proton intercalation was evidenced mainly using nuclear resonance spectroscopy.⁵ Nonetheless, the inserted chemical species (i.e., Mg(H₂O)_n²⁺, Mg(solvent, H₂O)_n²⁺, H⁺, H₃O⁺, H₂O or any combination of these) are still unclear because of lack of the crystal structure of the reduced phase.

In this work, the crystal structure of the magnesium-inserted phase of α-V₂O₅—electrochemically reduced in 0.5 M Mg(ClO₄)₂ + 2.0 M H₂O in acetonitrile—was solved for the first time by the ab initio method using a powder synchrotron X-ray diffraction data. An orthorhombic structure (P2₁2₁2 space group; a = 11.512 Å, b = 10.5483 Å, and c = 4.3681 Å) was identified; the structure was tripled along the b-axis from that of the pristine V₂O₅ structure (Figure). Interestingly, no appreciable densities of elements were observed other than vanadium and oxygen atoms in the electron density maps, suggesting the inserted species should have very low occupancies in large cavity sites surrounded by oxygen atoms in the structure. Examination of the interatomic distances around the cavity sites suggested that H₂O, H₃O⁺, or solvated magnesium ions are too big for the cavities, leading us to conclude that the intercalated species are single Mg²⁺ ions or protons. The general formula of magnesium-inserted V₂O₅ is Mg_0.17H_yV₂O₅, (0.66 ≤ y ≤ 1.16), where the content of Mg was determined from ICP analysis and the proton content, y, could not be determined precisely from the reduction capacity because water reduction on the surface of the electrode seemed to occur simultaneously. The 3D BVS difference maps were used to locate the intercalated ion sites. The isosurfaces for protons and Mg ions showed a conduction pathways in the structure, and suggested a number of possible sites for each of the ions, being distributed without overlapping. However, the too many possible sites made it difficult to locate a specific position of each ion. Thus, density functional theory calculations were carried out to locate the most plausible atomic sites of the magnesium and protons, enabling us to complete the structure modeling. Finally, Rietveld refinement was performed. This work provides an explicit answer to the age-old question about Mg intercalation into α­-V₂O₅, and reveals the nature of co-intercalation of magnesium and proton.

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